Soft water, especially when full of air, or when containing organic matter, acts upon lead in such a way that some of it is taken up in solution, and the water is poisoned. Vitiated or impure air acts upon lead in a somewhat similar manner. Pure water, not containing air, does not act upon pure lead. When the water contains much oxygen, the lead is oxidised; and oxide of lead, a highly poisonous substance, is to some extent soluble in water. If there is much carbonic acid present it converts some of the oxide into carbonate of leal, which is almost insoluble and therefore comparatively harmless. The waters which act most upon lead are the purest and most highly oxygenated, also those containing organic matter - nitrites, nitrates, and chlorides. The waters which act least upon lead are those containing carbonate of lime and phosphate of lime, in a less degree sulphate of lime. Some of these form a coating on the inside of the pipe which protects it from further action. Some vegetable substances contained in water, peaty matter for example, also protect the pipe, by forming an internal coating upon it. It appears therefore that hard waters, containing (as they generally do) carbonate of lime, do not readily affect lead.
Soft waters, such as rain water, and water obtained by distillation - water polluted with sewage-water in tanks having a muddy deposit - may all become poisoned when in contact with lead. The mud of several rivers, even the Thames, will corrode lead, probably from the organic matter it contains, but it does not necessarily follow that any lead has been dissolved in the water. Bits of mortar will also corrode lead. Vegetables and fatty acids arising from fruit and vegetables, cider, sour milk, etc, also act upon lead.
(1) Prof. Emerson Reynolds has described a process for the protection of lead against corrosion, which is done by coating it with a film of sulphide of lead. He recommends the following method: - Take 4 dr. solid caustic soda, dissolve it in 3 1/4 pints water, and add to the liquid 4 1/4 dr. nitrate of lead, or an equivalent of other lead salt, with 62 fl. dr. water; raise the temperature of the mixture to 194° F. (90° C). It sufficient lead salt has been added, the liquid will remain somewhat turbid after heating, and must then be rapidly strained or filtered through asbestos, glass-wool, or other suitable material, into a convenient vessel. The filtered liquid is then well mixed with 25 fl. dr. hot water, containing in solution 1 dr. sulpho-urea or thio-carbamide. If the temperature of the mixture be maintained at about 158° F. (70° C), deposition of sulphide of lead or galena, in the form of a fine adherent film or layer, quickly takes place on any object immersed in or covered with the liquid, provided the object be in a perfectly clean condition and suitable for the purpose.
When the operation is properly conducted, a layer of galena is obtained, which is so strongly adherent that it can be easily polished by means of the usual leather polisher, it is not neccssary to deposit the galena from hot liquids, but the deposition is more rapid than from cold solutions.
(2) Dr. Percy observes that in the collection of the Museum of Practical Geology, in London, is a number of very thin sheets of lead, coated with bands of varied and extremely bright colours. Although the atmosphere has had free access to these sheets for about 30 years, the colours are as bright as they were at first. The sheets were prepared at Beaumont's smelting works, by dexterously skimming in the process of desilverizing lead by Pattinson's process. The colours are certainly caused by excessively thin films of oxide of lead of various thickness.
(3) Applying an internal bituminous coating is said to be successful.